Dilational rheological properties of sulphobetaines at the water–decane interface: Effect of hydrophobic group

• The benzyl substituted alkyl sulphobetaine occupies larger interfacial area than that of linear sulphobetaine.
• The sulphobetaines can form more compact adsorption films with higher moduli than traditional surfactants.
• The benzyl substituted alkyl sulphobetaine shows more elastic nature due to the rigidity of benzene ring.
• The electrolyte NaCl has little effect on interfacial tension but can reduce interfacial modulus.

The interfacial tensions and dilational properties of adsorbed films of two zwitterionic surfactants with different hydrophobic groups, alkyl sulphobetaine (ASB) and benzyl substituted alkyl sulphobetaine (BSB), at the decane–water interface have been investigated by the drop shape analysis method. The influences of dilational frequency and bulk concentration on the interfacial dilational modulus and phase angle were expounded. The effects of electrolyte NaCl on the interfacial tensions and dilational properties have also been researched. The experimental results show that the similar CMC and IFTCMC values for ASB and BSB indicate their similar interfacial activity. The Amin value of BSB is larger than that of ASB, which is due to the steric effect of benzene ring in BSB molecule. The dilational data show that betaines can form compact adsorption films due to their electric neutrality in experimental condition, which results in the higher maximum values of dilational moduli for both BSB and ASB than those of common surfactants. Moreover, the whole hydrophilic part, sulfonic group, hydroxyl and ammonium group, can response to the area variations by change the orientation between parallel to the interface and introduce to the aqueous instead of diffusion-exchange process, which can enhance the elasticity of the film. The electrolyte NaCl has a function of accelerating the diffusion-exchange process of surfactant molecules. As a result, the dynamic interfacial properties such as dilational properties show obvious variations, while the static interfacial property such as interfacial tension has little change after the addition of 1% NaCl.

The whole hydrophilic part, sulfonic group, hydroxyl and ammonium group, can response to the area variations by change the orientation between parallel to the interface and introduce to the aqueous instead of diffusion-exchange process, which can enhance the elasticity of film.Figure optionsDownload as PowerPoint slide